The major empirical objective of this research is to systematically document repercussions associated with persistent deviations from a regular 16 hr wakefulness - 8 hr monophasic nocturnal sleep schedule among the young adult, utilizing (a) electrographic (EEG-sleep) criteria, (b) sensorimotor performance tasks, and (c) relevant psychobiologic indices reflecting levels of daytime alertness. A premise guiding this investigation conceives sleep propensities and overt waking behavioral response capacities as continuously fluctuating phases in the 24-hr cycle of existence. As a strategy concerned with further determining the extent to which behavioral adaptation for the young adult is contingent upon maintaining a stable, socially acceptable, 24-hr sleep-wakefulness schedule, various indices reflecting daytime alertness in specific functional modalities are being compared among separate constituencies (Ns equals 30) of ostensibly healthy subjects comprised by (a) controls who habitually sleep from 12:00-8:00 a.m. for 7-8 hrs, (b) irregular sleepers whose retiring and awakening times continuously fluctuate by about 2-4 hr within the vicinity of 12-8 a.m., and (c) chronic insomniacs whose reported difficulty in falling asleep is coupled with erratic nocturnal sleeping habits. An impetus for the rigorous application of performance tasks to this research is based on previous systematic observations that perceptual capacities, locomotion and conditioned reflex activity are among the most seriously impaired responses when customary 24-hr schedules become suddenly disrupted.